When anodization is performed on aluminum, an anodized layer which has a porous alumina layer in its surface is formed. Conventionally, anodization of aluminum has been receiving attention as a simple method for making nanometer-scale micropores (very small recessed portions) in the shape of a circular column in a regular arrangement. A base is immersed in an acidic electrolytic solution of sulfuric acid, oxalic acid, phosphoric acid, or the like, or an alkaline electrolytic solution, and this is used as an anode in application of a voltage, which causes oxidation and dissolution. The oxidation and the dissolution concurrently advance over a surface of the base to form an oxide film which has micropores over its surface. The micropores, which are in the shape of a circular column, are oriented vertical to the oxide film and exhibit a self-organized regularity under certain conditions (voltage, electrolyte type, temperature, etc.). Thus, this anodized porous alumina layer is expected to be applied to a wide variety of functional materials.
A porous alumina layer formed under specific conditions includes cells in the shape of a generally regular hexagon which are in a closest packed two-dimensional arrangement when seen in a direction perpendicular to the surface of the oxide film. Each of the cells has a micropore at its center. The arrangement of the micropores is periodic. The cells are formed as a result of local dissolution and growth of a coating. The dissolution and growth of the coating concurrently advance at the bottom of the micropores which is referred to as a barrier layer. As known, the size of the cells, i.e., the interval between adjacent micropores (the distance between the centers), is approximately twice the thickness of the barrier layer, and is approximately proportional to the voltage that is applied during the anodization. It is also known that the diameter of the micropores depends on the type, concentration, temperature, etc., of the electrolytic solution but is, usually, about ⅓ of the size of the cells (the length of the longest diagonal of the cell when seen in a direction vertical to the film surface). Such micropores of the porous alumina layer may constitute an arrangement which has a high regularity (periodicity) under specific conditions, an arrangement with a regularity degraded to some extent depending on the conditions, or an irregular (non-periodic) arrangement.
The voltage during anodization for controlling the thickness of the barrier layer and the interval between adjacent micropores is increased to a predetermined value with an appropriate time change rate (see, for example, Patent Documents 1 to 3). Patent Document 1 discloses that increasing the voltage relatively slowly leads to uniform formation of recessed portions. Patent Document 3 describes applying a voltage at a high frequency in order to increase the advance rate of the depth of the recessed portions.
When anodization is performed, bubbles of hydrogen are generated at a cathode immersed in an electrolytic solution. When the hydrogen bubbles are left without being removed, the working environment deteriorates. Patent Document 4 describes covering the cathode with a mesh cloth which allows passage of hydrogen bubbles to some extent, thereby controlling diffusion of the electrolytic solution.
An anodized layer which has such a porous alumina layer can be used for production of an antireflection element (see Patent Documents 5 to 8). The antireflection element utilizes the principles of a so-called moth-eye structure. A minute uneven pattern in which the interval of recessed portions or raised portions is not more than the wavelength of visible light (λ=380 nm to 780 nm) is formed over a substrate surface. The refractive index for light that is incident on the substrate is continuously changed along the depth direction of the recessed portions or raised portions, from the refractive index of a medium on which the light is incident to the refractive index of the substrate, whereby reflection of a wavelength band that is subject to antireflection is prevented. The two-dimensional size of a raised portion of an uneven pattern which performs an antireflection function is not less than 10 nm and less than 500 nm.
Providing an antireflection element on the surface of a display device for use in TVs, cell phones, etc., or an optical element, such as a camera lens, enables reduction of the surface reflection and increase of the amount of light transmitted therethrough. When light is transmitted through the interface between media of different refractive indices (e.g., when light is incident on the interface between air and glass), the antireflection technique prevents decrease of the amount of transmitted light which may be attributed to, for example, Fresnel reflection, and as a result, the visibility improves. The moth-eye structure is advantageous in that it is capable of performing an antireflection function with small incident angle dependence over a wide wavelength band, as well as that it is applicable to a number of materials, and that an uneven pattern can be directly formed in a substrate. As such, a high-performance antireflection film (or antireflection surface) can be provided at a low cost.
For example, Patent Document 6 discloses a method of producing an antireflection film (antireflection surface) with the use of a stamper which has an anodized porous alumina film over its surface. Patent Document 7 discloses the technique of forming tapered recesses with continuously changing pore diameters by repeating anodization of aluminum and a pore diameter increasing process. Patent Document 8 discloses the technique of forming an antireflection film with the use of an alumina layer in which very small recessed portions have stepped lateral surfaces.
As described in Patent Documents 5, 6, and 8, by providing an uneven structure (macro structure) which is greater than a moth-eye structure (micro structure) in addition to the moth-eye structure, the antireflection film (antireflection surface) can be provided with an antiglare function. The two-dimensional size of a raised portion of the uneven structure which is capable of performing the antiglare function is not less than 1 μm and less than 100 μm. The entire disclosures of Patent Documents 5, 6, and 8 are herein incorporated by reference.
Utilizing an anodized porous aluminum film as described above can facilitate the manufacture of a mold which is used for formation of a moth-eye structure over a surface (hereinafter, “moth-eye mold”). In particular, as described in Patent Documents 6 and 8, when the surface of the anodized aluminum film as formed is used as a mold without any modification, the manufacturing cost can be reduced. In this specification, the structure of the surface of a moth-eye mold which is capable of forming a moth-eye structure by means of transfer, for example, is herein referred to as “inverted moth-eye structure”.